Add support for ECS constraints

[andrewstucki]

What does this PR do?

This PR adds support for constraints in the ECS provider

Motivation

I was running a couple of Traefik containers as ECS instances using roughly the following task definition:

[
  {
    "cpu": 512,
    "essential": true,
    "image": "traefik:1.6.1-alpine",
    "memory": 256,
    "name": "ingress",
    "command": [
      "--constraints=tag==public",
      "--ecs.clusters=my-cluster",
      "--ecs.exposedbydefault=false",
      "--api"
    ],
    "portMappings": [
      {
        "containerPort": 8080,
      },
      {
        "containerPort": 80,
      }
    ]
  }
]

Other containers had docker labels a la:

    "dockerLabels": {
      "traefik.frontend.rule": "Host:myhost.mydomain.com",
      "traefik.enable": "true",
      "traefik.tags": "private"
    },

ECS wasn't using traefik.tags to filter out this container, or any other containers with mismatched constraints.

More

• Added/updated tests
• Added/updated documentation

[mmatur]

LGTM

[dtomcej]

LGTM

[ldez]

LGTM